The present invention is broadly applicable to an improved composition and process for treating ferrous-base substrates susceptible to corrosion to impart corrosion resistance to the surfaces thereof inhibiting the formation of rust spots during in-process operations prior to final surface treatment such as coating, lacquering, painting or the like. The benefits of the present invention are particularly adapted for the treatment of container bodies comprised of low-carbon steel sheet, commonly referred to as black plate, which are readily fabricated employing conventional cupping and draw and ironing press operations. Such drawn and ironed black plate container bodies possess a desirable light-gray shiny steel surface appearance which provides for an attractive package after subsequent coating with a clear organic lacquer and the imprintation of ink indicia on the exterior surfaces thereof.
The sequence for manufacturing black plate container bodies conventionally comprises uncoiling a black plate steel strip having a protective oil layer on the surfaces thereof to which further drawing lubricants are applied after which the strip passes through a cupping press forming a preliminary cup-shaped disc which is transferred to a draw and ironing press producing an elongated cup-shaped body. The draw and ironing press operation usually employs supplemental coolants such as water or dilute aqueous emulsions to facilitate the drawing operation. The fabricated container bodies are thereafter transferred to a trimmer in which the upper edge is trimmed whereafter the trimmed container body is subjected to a washer cycle containing multiple stages usually including a pre-washing stage in which water containing a low concentration of a cleaner is applied followed by a cleaning step in which an alkali cleaner of conventional strength is applied to remove the various contaminating lubricants, protective oils, coolants and other contaminating substances on the surfaces thereof. The cleaned container bodies are thereafter subject to one or a plurality of water rinse stages whereafter they are transferred to a dry-off oven for complete drying and thereafter are subjected to one or a plurality of lacquering steps and exterior decorative printing steps. Typically, the exterior surface of the container body is first provided with a base coat and/or a decorative ink printing of suitable indicia which after drying is followed by a conventional exterior can lacquer coating which is cured and followed by an interior can lacquer coating of the types conventionally employed which thereafter is also cured. Should the "covering power" of the chosen layer be poor, the resultant container body to be commercially acceptable must retain the shiny, light-gray metallic appearance visible or partially visible through the lacquer coated areas.
It has been observed in the manufacturing sequence of such black plate container bodies, that rust spots sometimes appear if too much water is retained at localized areas of the can during the drying stage such as the dome, lip or points of contact between containers necessitating scrappage or reworking thereof. Inadvertent stoppages of the production line in which the container bodies are retained in the washer stages for prolonged time periods have also occasioned unsightly rust spots or streaks which may be visible even through a base coat and to which coatings may poorly adhere or unevenly spread on the container surfaces rendering them commercially unacceptable.
The foregoing problems are overcome in accordance with the practice of the present invention by applying an aqueous acidic treating composition to the black plate container body as an integral stage of the multiple-stage washer sequence whereby in-process rusting of the container bodies is prevented as a result of inadvertent line stoppages or excessive localized water concentrations in the dry-off oven thereby preserving the desirable shiny, light-gray surface appearance until final protection is provided by a lacquer or subsequent surface coating operation.